High purity gas delivery systems, particularly those transporting gases such as HCl, HBr, etc., which are used in the wafer fab, are usually made of 316 L stainless steel tubings of approximate elemental composition 18% chromium, 12% nickel, 2.5% molybdenum, and 2-3% of manganese. 316 L Stainless steel displays improved corrosion resistance in comparison to other steels as a consequence of low content of carbon which avoids the precipitation of chromium carbide at grain boundaries. This kind of tubing receives also surface treatments such as electropolishing which improve corrosion resistance and other required properties.
However, stainless steel comprising a low content of carbon and being further surface treated loses its corrosion resistance properties during welding as disclosed by H. H. Uhlig & R. W. Revie in "Corrosion and Corrosion Control", by John Wiley & Sons Third Ed (1985). This arises from two main reasons: the first reason is the destruction of the passivated steel surface, and the second reason is the formation of heat affected zone, modifying the bulk properties of the stainless steel.
Moreover, a corrosion sensitization of the zone surrounding the welded area appears as a consequence of the emission of manganese fumes (due to manganese high partial pressure) during the melting of the bead and deposition of manganese particles on the inner surface, wherein up to 50% of the inner surface of the tubing at 5 mm downstream the welding is covered by manganese particles. These manganese fumes act as specific corrosion initiators in corrosive gas environment as disclosed e.g. by S. Miyoshi, A. Ohki, K. Kawada, M. Nakamura, T. Watanabe, S. Takahashi and T. Ohmi, in the article entitled "Ultra Clean Welding Technology without Accompanying Corrosion", in 18th workshop on Ultra Clean Technology, Ultra Clean Society, Tokyo, p. 204 (1992). Usually, manganese fumes are produced by the molten metal during welding. The fumes are present on both sides of the tube. On the inner side of the tube the inert gas which flows through the tube during said welding entails the deposition of manganese particles downstream the welding place, which later on creates corrosion spots by interaction with the corrosive gases such as HCl, HBr, etc.
It has been also proposed to improve corrosion resistance of the welding bead by adding a corrosion resistant metal in the bead in the form of a wire or an electrode as disclosed e.g. in Japanese Patent application 78/040656 entitled "Core wire for welding rod for welding stainless steel-consist of austenite and ferrite phases", or in Japanese Patent application 82/013399 entitled "Core wire for welding rod for welding stainless steel--consist of austenite and ferrite phases".
A similar improvement using a coating process has been disclosed in Japanese Patent application 78/018444 entitled "Welding process for stainless steel pipes or plates--having surface layer of chromium and nickel, with deposition of corrosion resistant layer on weld zone". The coatings are either deposited before the welding (coated tubes) or after.
In the case of small diameter tubings e.g. 1/8 inch, 1/4 inch or even larger diameter, the welding technique required for a strong and smooth joining is usually TIG or Tungsten Inert Gas welding, involving a non-melting electrode. Therefore, the techniques disclosed hereabove cannot be applied.